Can a Metal Form an Anion?

Yes. “Metal” describes a broad class of elements, not a rule that an atom can only lose electrons. Some metal atoms bind an extra electron, and carefully chosen chemical environments can stabilize metal-containing anions. The answer depends on what you mean by “a metal”: an isolated atom, a metal atom in a compound, or a piece of bulk metal.

An isolated metal atom can bind an electron

For a gas-phase atom, the process is

$$\mathrm{M(g) + e^- \rightarrow M^-(g)}$$

The key question is whether the extra electron is bound. The electron affinity is commonly defined as the energy released when a neutral gas-phase atom gains an electron. With that convention, a positive electron affinity means the anion is lower in energy than the neutral atom plus a free electron, so formation is energetically favorable in the gas phase. Some references instead report the energy change for the attachment process, giving the opposite sign; check the convention when comparing tables.

Many metal atoms have positive electron affinities. For example, neutral sodium has the configuration $[\mathrm{Ne}]3s^1$. Adding one electron gives sodium anion, $\mathrm{Na^-}$, with configuration $[\mathrm{Ne}]3s^2$. Sodium’s gas-phase electron affinity is about $0.548$ eV, so the attachment releases energy.

This does not mean sodium normally behaves as an anion in everyday chemistry. In familiar reactions, sodium commonly loses its $3s$ electron to form $\mathrm{Na^+}$. That is a statement about what is favored in those chemical settings, not a prohibition on forming $\mathrm{Na^-}$ under other conditions.

Worked example: the energy for forming sodium anion

Using the positive-energy-released convention, sodium’s electron affinity is approximately $0.548$ eV per atom. To express this per mole, use $1$ eV per particle $\approx 96.485$ kJ/mol:

$$\begin{aligned} 0.548\ \mathrm{eV/atom} \times 96.485\ \mathrm{kJ\,mol^{-1}\,eV^{-1}} &\approx 52.9\ \mathrm{kJ/mol}. \end{aligned}$$

Thus, forming gas-phase $\mathrm{Na^-}$ from gaseous sodium atoms and electrons releases about $52.9$ kJ per mole of atoms. This is a gas-phase energy comparison; it does not by itself predict whether a particular sodium compound or solution will form.

Metal anions in compounds are a separate case

A compound can contain a metal with a negative formal oxidation state. For example, in the carbonyl anion $[\mathrm{Co(CO)_4}]^-$, carbon monoxide is treated as a neutral ligand, so formal charge bookkeeping assigns cobalt oxidation state $-1$.

An oxidation state is a useful accounting convention, not a direct measurement of the charge sitting on one atom. In a real molecule, electrons are distributed across bonds and orbitals. So a metal’s negative oxidation state does not necessarily mean the compound contains a localized, isolated $\mathrm{Co^-}$ ion.

Some unusual salts called alkalides contain alkali-metal anions, such as $\mathrm{Na^-}$, stabilized by carefully selected molecular hosts and counterions. These are specialized systems, not the usual behavior of sodium salts in water. Their existence shows that chemical surroundings can make anionic forms possible even when those forms are uncommon in ordinary conditions.

What about a piece of metal?

A bulk metal is not usually described as a collection of discrete metal anions. Its electrons occupy extended electronic states across the solid. Adding an electron to a metal involves the solid’s electronic structure and the conditions at its surface; it is not the same question as whether one isolated atom has a positive electron affinity.

In principle, electrons can be supplied to a material, but whether they remain in the material depends on its energy levels, surroundings, and how the charge is compensated. The simple gas-phase electron-affinity test applies to isolated atoms, not directly to a bulk sample.

Common mistakes

• Assuming every metal must lose electrons. Metals often form cations, but that is a common chemical tendency rather than a universal rule.
• Treating electron affinity and oxidation state as identical. Electron affinity concerns adding an electron to an isolated gas-phase atom; oxidation state is formal bookkeeping within a substance.
• Assuming a negative ion must be stable in every environment. An anion may be stable as a gas-phase ion yet react readily in a solvent or compound, or vice versa.
• Calling every electron-rich metal a metal anion. A bulk metal’s delocalized electrons are not the same as discrete $\mathrm{M^-}$ ions.

To decide whether a particular metal can form an anion, specify the setting first, then check the atom’s electron affinity or the stability of the proposed compound under those conditions.